1. Field of the Invention
The present invention relates to a power conversion device that directly generates an arbitrary frequency from an AC power supply.
2. Description of Related Art
A direct AC power conversion device connects each phase of an AC power supply to each phase on an output side through a bidirectional switch, and directly converts the AC power supply into arbitrary voltage and frequency.
The direct AC power conversion device requires a unique switching operation in order to forcibly switch a current flowing to the switch. The switching operation will be hereinafter referred to as commutation.
Examples of a method of generating commutation include a method of generating commutation on the basis of an output current direction signal obtained by an output current direction detection circuit and the like. The commutation is generated in a logic circuit on the basis of a PWM signal. The commutation performs a switching operation so as to be delayed by a switching time of a semiconductor device serving as a switch.
For this reason, an error is generated between a voltage command of an output voltage which is to be originally output and an output voltage which is actually output. This error depends on the state of a power supply, the direction of an output current, the magnifitude relation of a power supply voltage, and the like.
With respect to such a technical problem, a correction method is suggested which detects the direction of a current or the state of a voltage in advance, and corrects a command voltage to correct the error in the output voltage.
For example, JP-A-2005-348479 discloses that an error due to four-step commutation is avoided by previous correction to a command and by reduction of the number of commutation steps around zero.